oil+resin hydraulic pump
Hydraulic pump based on thermal expansion of the resin in the oil

Solution

Polyamid resin  with oil  in the closed volume create high pressure during heating. Two similar volumes being heated or cooled in turn can move hydraulic pump.
 Cooling is performed by the  cold water   sucked from the well. Polyamid 6 resin has a price approx 3-4 USD per kilo. Used and filtered motor oil may be used for futher price reduction.  There is no need in direct sunlight. If the outdoor temperature is higher that  12-15grad C ( water in the well) , machine will run.